One source, a board member of a big Valley company, tells us: "There are a whole set of companies, starting with Square, that are hot that people believe are going to be better places to earn money. They don't believe Facebook is going to do anything.

One top Valley recruiter tells us Facebook is just dealing with a "pickup in attrition" expected of "a larger, older company."

She was trying to be nice, but…ouch.

Another theory, which a senior Facebook executive in human resources shared with a source of ours, is that Facebook grew its head count too quickly. Facebook grew from hundreds of employees to thousands in a very short period of time. This created all sorts of organizational flaws, but the company's exploding private market valuations covered them up. Morale is great when you're going from $50 billion to $100 billion a year. It can get very bad when you go from $100 billion to $50 billion in five weeks.

Our source on a big Valley board says Facebook will have a hard time correcting these problems because it's not issuing as much stock to employees as it used to. Stock compensation comes off the bottom line, and Facebook is trying to prove itself as a profitable, fast-growing company at the moment.